1. Field of the Invention
An apparatus and method for accomplishing the formation of an elongated thin-walled tubular product such as but expressly not limited to a medical catheter wherein an extremely small diameter bore is maintained along with a thin wall of substantially consistent dimension throughout a majority of the length of the tubular product utilizing an injection molding technique and forcing a flow of melt into a mold cavity in surrounding relation to a central core pin itself maintained in a substantially centered relation to the mold cavity through the application of tension being applied thereto.
2. Description of the Prior Art
There is a need in the art of molding plastic products to be able to mold a plastic part having a relatively small diameter hole extending therethrough. An example of such a product is a medical catheter comprising a jacket or sleeve of plastic material through which a needle is to be inserted. Especially in the field of medical products, it is important to exert great care in maintaining the constant diameter of the central hole extending through a catheter product since this has the effect of maintaining a substantially straight shank in the finished tubular product. Also, the product must be constructed to have a consistent flex pattern and avoid "kinking" when bent or flexed.
There are of course numerous other situations in which a relatively fine, small diameter hole is required to be formed through a tubular plastic molded part whether the hole is substantially coaxial with the part or not, or whether the part is cylindrical or not. Generally, the term "tubular" as used herein refers to parts which have at least one fine hole therethrough. Oftentimes, such molded plastic parts are required to have a thin wall of necessarily constant thickness with a longitudinally extending channel, having an exceptionally small diameter extending therethrough.
Previously, especially in the art of injection molding, when plastic is melted, and forced to flow into a cavity about a fine, small diameter core, the pressure of injection forcing the melt into the mold cavity if not adequately balanced during the molding operation tends to cause the thin diameter core to be displaced from its substantially centered, straight line configuration relative to the central axis of the mold cavity. This displacement is of course undesirable since a straight line configuration of the central hole as well as the shank portion of the product is required. Such displacement of the core pin relative to the interior surface of the mold cavity is also frequently caused by what may be considered an imbalance in the flow characteristics of the hot melt as it enters the mold cavity and passes along the length thereof in substantially surrounding relation to the core pin. Such imbalance may be caused by a number of factors but also has the tendency to cause uneven forces to be exerted on the core pin tending to cause its displacement from the aforementioned and preferred straight line configuration. For these reasons, the prior art has generally been limited to relatively short tubular lengths in relation to the diameter of the hole through it. If, for example, the tubular product is to be relatively long, which is quite common in the medical industry, a small hole, especially a hole in a thin walled tube is extremely difficult to make with any great degree of accuracy. In addition, different materials provide different problems.
In manufacturing the type of product set forth above, it is often required to make small diameter holes of about 0.020 inches in a tubular product of plastic having a longitudinal dimension of an inch or more with an outside diameter of approximately 0.032 inches. It is readily apparent therefore that the wall of such a tubular product is extremely thin and in the range of about 0.006 inch in thickness. Oftentimes it is desired that the hole or passageway through the molded plastic part be in the range of 0.002 inches to about 0.100 inch in diameter.
Accordingly, in the formation of the type products referred to above, a primary difficulty has been that the core pin must be of an extremely small diameter and therefore subject to pressures of the inflowing melt forced into the mold cavity utilizing injection molding techniques. The pressure or force of the incoming melt has a tendency to exert a force on the very thin core pin resulting in a displacement from its straight line configuration within the mold cavity and the production of a deflective thin walled tubular product.
Another problem generally associated with the injection molding technique of products of the type referred to herein is the breakdown in the integrity of the plastic material utilized to form the tubular product when the material is forced to travel along what may be referred to as a circuitous path from the point of leaving the injection nozzle to the point of entering and passing along the length of the mold cavity. This is particularly true when the plastic material, after being melted, is forced to travel along a circuitous path or a path including one or more right angles at a relatively high speed. Attempts to slow the speed of injection of the inflowing melt however has met with little success especially in the formation of thin walled tubular products. This is primarily due to the fact that the melt rapidly cools and therefore hardens as it travels along the length of the mold cavity especially at stow speeds. Accordingly, when using injection molding techniques at such stow speeds, attempts have been made to raise the temperature of the plastic material to a somewhat higher than normal temperature, prior to injection to overcome the problems of the prior art. This higher temperature results in a degradation of many plastic materials which has obvious disadvantages in the making of thin walled tubular products.
As set forth above, various materials utilized in the formation of thin walled tubular products present specific problems when utilizing conventional injection molding techniques. The referred to plastic material Teflon cannot be used in injection molding Incorporating circuitous paths under high speed conditions. To the contrary, the injection molding of Teflon along right turn paths or circuitous paths require a slow injection speed. However, it is recognized that in the formation of thin walled tubular products, high speed injection is necessary to accomplish filling of the mold cavity before cooling of the melt.
Other prior art techniques and apparatus for the formation of relatively long, thin walled tubular products include the use of drilling with an extremely fine drill or a laser beam. However, such techniques seriously limit the length of the tube which may be created. Also, it is of course well known to extrude a tube and draw the tubular product to a smaller diameter. This technique however presents limitation as to the configuration of the tube wall and the latter type product is limited to a hole or external configuration which is cylindrical. Generally, such prior art extrusion techniques would not be practical when it is desired to produce such products with a multi-stepped wall configuration such as when a hub or angeled wall tip is desired on the product.
Accordingly, there is a need in the industry for an apparatus and method of producing elongated thin walled tubular products such as, but expressly not limited to, medical catheter type products, preferably using an injection molding technique in a manner which will overcome problems associated with the displacement of a relatively fine core pin relative to its preferred coaxial positioning in the mold cavity in which the tubular product is formed.